Integrated circuits fabricated on silicon wafer substrates are electrically connected to printed circuit boards by wire bonds. The wire bonds are very thin wires—around 25 to 40 microns in diameter—extending from contact pads along the side of the wafer substrate to contacts on the printed circuit board (PCB). To protect and strengthen the wire bonds, they are sealed within a bead of epoxy called encapsulant. The wires from the contact pads to the PCB are made longer than necessary to accommodate changes in the gap between the PCB and the contact pads because of thermal expansion, flex in the components and so on. These longer than necessary wires naturally form an arc between the contact pads and the PCB. The top of the wire arc is often about 300 microns above the contact pads although some wire bonding may extend even higher. As the name suggests, the encapsulant needs to encapsulate the full length of the wire so the encapsulant bead will extend 500 microns to 600 microns proud of the contact pads.
The integrated circuit fabricated on the silicon wafer is often referred to as a ‘die’. For the purposes of this specification, the term die will be used as a reference to an integrated circuit fabricated on a wafer substrate using the well known etching and deposition techniques commonly used in semiconductor fabrication. If the die is purely an electronic microprocessor, there is little need to keep close control of the encapsulant bead dimensions. However, if the die is a micro-electro mechanical systems (MEMS) device with an active upper surface, it may be necessary or desirable to bring the active surface of the die onto close proximity with another surface. One such situation applies to inkjet printheads. The proximity of the print media to the nozzle array influences the print quality. Similarly, if a cleaning surface is wiped across the nozzles, the bead of encapsulant can hamper the wiping contact.
Wirebonders are automated devices that weld small lengths of wire from conductors on the PCB to the contact pads on an integrated circuit die. Wire is fed through a bonding tool that uses some combination of pressure, heat and/or ultra-sonic energy to attach the wire to the bond pads via a solid phase welding process. The two most common types of wire bonder are referred to as wedge bond and ball bond. These refer to the bonding tool and the configuration of the wire bond itself. With both types of wirebonders, the individual wire bonds extend in an arc from the bond pad on the integrated circuit (IC) die to the conductor on the PCB. This is because wires from the contact pads to the PCB are made longer than necessary to accommodate changes in the gap between the PCB and the bonds pads due to thermal expansion, flex in the components and so on.
Wedge bonders have the advantage of a lower loop height. Ball bonders weld a ball of metal to a contact with the wire extending away from the ball at right angles to the contact surface. In contrast, the wedge bonder presses the side of the wire against the contact and so incident angle of the wire to the plane of the contact is lower. Hence the loop height is also lower. However, there is a weak spot at the transition point between the wire welded to the contact and the wire extending away from the contact at an angle. This point is often referred to as the heel of the wire bond and is significantly strain hardened from bending and the ultra-sonic welding process. The metal becomes brittle and less resistant to crack propagation. The localized deformation caused by the wedge is a stress concentration that provides a crack initiation site and fatigue failure occurs quickly with thermal cycling.
The bead of encapsulant reinforces the wire but the difference in thermal expansion between the wire and the underlying support is still sufficient to cause bending at the heel and ultimately fatigue failure.
Accurately depositing the bead of encapsulant on the bond pads is problematic. One commonly used technique for depositing the encapsulant involves extruding it from a needle directly onto the line of wire bonds. The encapsulant volume and placement on the die is not very accurate. Variations in the pressure from the pump or slight non-uniformities in the speed of the needle cause the side of the bead contacting the active surface to be reasonably crooked. As the side of the bead is not straight, it has to be generously spaced from any active parts on the active surface to comfortably accommodate the perturbations. Spacing the electrical contacts away from the active portions (say for example, inkjet nozzles) of the active surface uses up valuable wafer real estate and reduces the number of dies that can be fabricated from a wafer disc.
“Jetting” is another common encapsulant deposition technique. A nozzle ejects relatively large drops (10 to 50 pico-liters) of epoxy encapsulant directly onto the wire bonds. This is a more precise method of deposition in terms of dimensional accuracy. However, jetting encapsulant down onto the wire bonds can produce bubbles of trapped air inside the bead. When the epoxy is cured, the heat increases the pressure in the bubbles and cause cracks in the epoxy. This can break or expose the wires which then fail prematurely.
The air bubbles are prone to form when the surface beneath the wire bonds has a complicated topography. For example, deep trenches or stepped formations can present shapes and geometries that do not completely fill with the uncured epoxy as it is flows over the wire bonds and into the underlying surface. In surface geometries with a section that is narrower than the meniscus curvature of the uncured epoxy, the epoxy flow pins at the narrow section and fails to wet the entire underlying surface thereby trapping an air bubble.
Another problem associated with jetting encapsulant is the generation of satellite drops that break off from the main drops of encapsulant. The satellite drops are several orders of magnitude smaller than the main drops and so susceptible to misdirection from air turbulence. With normal integrated circuit dies, misdirected satellite drops are of little consequence. However, if the die as an active surface such as an inkjet printhead die, the small satellite drops of epoxy can have detrimental effects on the operation of any MEMS structures.
In light of the widespread use of inkjet printheads, the invention will be described with specific reference to its application in this field. However, the ordinary worker will appreciate that this is purely illustrative and the invention is equally applicable to other integrated circuits and micro-device (such as lab-on-a-chip devices) that are wire bonded to a PCB or other support structure.